Ultrastable particle-stabilized foams and emulsions

ABSTRACT

Described is a method to prepare wet foams exhibiting long-term stability wherein colloidal particles are used to stabilize the gas-liquid interface, said particles being initially inherently partially lyophobic particles or partially lyophobized particles having mean particle sizes from 1 nm to 20 μm. In one aspect, the partially lyophobized particles are prepared in-situ by treating initially hydrophilic particles with amphiphilic molecules of specific solubility in the liquid phase of the suspension.

TECHNICAL FIELD

The present invention belongs to the field of foam formation, inparticular to the field of stable foam formation.

BACKGROUND ART

The long-term stability of foams is an essential requirement in a widenumber of applications ranging from food and cosmetics to biomedicalimplants and engineering low-weight structures. Foams are extensivelyused as an end product in food and cosmetics, where the long-termstability is essential to keep desired physical and chemical propertiessuch as texture and rheological behavior [1, 2]. Well-established andemerging applications that use foams as an intermediate structure toproduce macroporous materials are also widely spread in the engineeringfield to fabricate thermal insulating materials and low-weightstructures [3-5], as well as in medicine to produce artificial implantsand scaffolds for drug delivery and tissue engineering [6, 7]. As anintermediate material, the foam has to be stable enough to allow for thefabrication of structures with tailored porosity and pore sizedistribution.

However, foams are inherently thermodynamic unstable systems which tendto undergo rapid coalescence and disproportionation of bubbles, due tothe markedly high interfacial energy associated with the gas-liquidinterface.

The state-of-the-art method to inhibit the coalescence anddisproportionation of bubbles in a foam is to use biomolecules (e.g.proteins and lipids) or long-chain surfactants (e.g. soaps anddetergents), which adsorb at the gas-liquid interface reducing the foamoverall free energy. However, since the adsorption of these molecules atthe interface is most often a reversible process, no long-term stabilitycan be achieved by this means. A practical solution to this problem hasbeen the use of gelling agents to set the foam structure beforecoalescence and disproportionation takes place. This has also beenaccomplished by solidifying the foam liquid media (lamellas). Most ofsuch setting processes are triggered by temperature changes, which limitthe fixing mechanism to relatively thinned cross-sections where nosignificant temperature gradients are developed. Alternative settingmechanisms based solely on chemical reactions at the foam liquid mediaare also possible, but are either very specific for a given foam systemand contain often toxic reactants.

Thus, there exists still a need for foams with improved long-timestability as well as means suitable to achieve such long-lasting foams.

In addition to surface active molecules, it was only recently recognizedthat partially-hydrophobic particles can also stabilize air bubbles insurfactant-free diluted suspensions [8-14]. Similarly to surfactantmolecules, the adsorption of colloidal particles onto a gas bubblesurface lowers the overall free energy of the gas-liquid interface. Thereduction of the total free energy upon particle adsorption is achievedby replacing part of the gas-liquid interfacial area with solids, ratherthan reducing the interface tension as in the case of surfactantmolecules [8, 9, 14]. The wetting properties of the adsorbing particledetermine its position at the interface and therefore the amount oftotal gas-liquid interfacial area replaced. Particles exhibitingintermediate hydrophobicity (contact angle θ close to 90°) can replace alarge area of the gas-liquid interface and thus are the most efficientin reducing the overall interfacial free energy. However, theinterfacial adsorption of submicron-sized particles displaying contactangles as low as 20° can already reduce the interface free energy bymore than a few hundred kTs, implying that particles are irreversiblyadsorbed at the air-water interface even for slightly lyophobizedparticle surfaces [14].

The stabilization of air bubbles with partially lyophobic particlesalone has been so far restricted to model experiments and to a fewobservations of single air bubbles on thin top layers in dilutedsuspensions [8, 9, 14].

DISCLOSURE OF THE INVENTION

Hence, it is a general object of the invention to provide a simple,general and preferably also inexpensive method to prepare wet foams, inparticular high-volume wet foams, exhibiting unprecedented long-termstability wherein the whole of the suspension is homogeneously foamed.

It is another object to provide means for preparing long-term stablefoams.

Now, in order to implement these and still further objects of theinvention which will become more readily apparent as the descriptionproceeds, the method to prepare wet foams exhibiting long-term stabilityis manifested by the features that colloidal particles in a suspensionare used to stabilize the gas-liquid interface, said particles beingpresent in amounts of at least about 1% v/v referred to the volume ofthe suspension, and the whole of said suspension being homogeneouslyfoamed.

The particles used to stabilize the gas-liquid interface are partiallylyophobic or lyophobized or their behavior is accordingly tuned bychanging the properties of the liquid media.

Long-time stability as used herein, in general means at least 30minutes, preferably 2 days, more preferred 1 week, whereby wet foams ofthe present invention can have stabilities of up to one year or more.

The terms lyophobic and lyophobized as used here designates particlesthat are hydrophobic and hydrophobized, respectively, and particles thatare metal melt repelling in case of metal foams.

The terms hydrophobic as used herein means miscible with water inamounts of up to 1% v/v referred to the total volume of the mixture.

The term hydrophilic in particular includes e.g alcohols and glycols.

The terms partially lyophobic particles, partially lyophobizedparticles, partially hydrophobic particles and partially hydrophobizedparticles herein are used for particles obtained by initiallyhydrophobic/lyophobic particles that have been partiallyhydrophilized/lyophilized and initially hydrophilic/lyophilic particlesthat have been partially hydrophobized/lyophobized.

Where aqueous systems are concerned, the terms lyophilic, lyophilized,lyophobic, lyophobized are used synonymous to hydrophilic,hydrophilized, hydrophobic, hydrophobized.

The partial lyophobization can be achieved in different ways, namely

-   -   by treating a hydrophilic surface with a specific amphiphilic        molecule,    -   rendering the behavior of hydrophobic particles more hydrophilic        by tuning the hydrophilicity of the solvent of the suspension to        be foamed,    -   by treating a hydrophobic surface with a specific amphiphilic        molecule

The high stability achieved with this new method stems from theirreversible nature of the adsorption of said partially-lyophobic orpartially-lyophobized particles at the bubble surface. The stabilizingcolloidal particles are e.g. initially hydrophilic and, preferablyin-situ, partially hydrophobized through the adsorption of specificamphiphilic molecules on the particle surface.

As a general rule, the specific amphiphilic molecules used to in-situpartially hydrophobize initially hydrophilic particles should be able toreduce the surface tension of an air-water interface to values lower orequal than 65 mN/m for concentrations lower or equal than 0.5 mol/l andhave a solubility in the liquid phase of the suspension given by thefollowing equation:

${{SOLUBILITY}\left\lbrack {{mol}\text{/}l} \right\rbrack} \geq {m \cdot \frac{\phi}{1 - \phi} \cdot \rho_{powder} \cdot S_{A}}$m = 4 ⋅ 10⁻⁸[mol/m²] ϕ:  Solids   Loading  [−]ρ_(power):  Density  of   powder   [g/l]S_(A):  Surface  Area  of  Powder   [m²/g]

The amphiphilic molecules consist of a tail (designated below as R)coupled to a headgroup. The tail can be aliphatic (linear or branched)or cyclic (alicyclic or aromatic) and can carry substituents. Suchsubstituents are e.g. —C_(n)H_(2n+1) with n<8, —OH, —NH₃, etc. Preferredtails are optionally substituted linear carbon chains of 2 to 8 carbonatoms. The headgroups that are coupled to the tail preferably are ionicgroups. Examples of possible headgroups are specified in Table 1 below,wherein the tail is designated as R.

For hydrophilization of initially hydrophobic particles, the amphiphilicmolecules suitable for in-situ surface hydrophilization have a criticalmicelle concentration (CMC) higher than 10 μmol/l and/or they have asolubility higher than 1 μmol/l.

TABLE 1

phosphates X: H, C_(n)H_(2n+1) (n < 7), alkaline metals

phosphonates X: H, C_(n)H_(2n+1) (n < 7), alkaline metals

sulfates

sulfonates R—OH alcohols

amines X: H, C_(n)H_(2n+1) (n < 7)

amides

pyrrolidines

gallates

carboxylic acids or corresponding salts. Some preferred examples are:

or corresponding salts.

Dependent on the charge of the surface to be coated either a negativelycharged headgroup is chosen or a positively charged headgroup. For e.g.Al₂O₃, a negatively charged headgroup is suitable at low pH conditions,i.e. pH lower than the isoelectric point, here pH<9, in particular pH4-5. The above mentioned headgroups and further similar groups can beused to modify a broad variety of particles, in particular smallparticles such as metal oxides, salts and metals.

Due to the high solubility and critical micelle concentrations of theshort amphiphiles, a high overall particle surface area can be coveredbefore insoluble micelles and clusters are formed. Therefore, anenormous number of surface-modified particles can be produced within thefoam liquid media and used as stabilizers of the gas-liquid interface.

Surface modification can be achieved through the physical or chemicaladsorption of negatively or positively charged amphiphile molecules ontoa suitable, preferably an oppositely charged surface leaving thehydrophobic tail in contact with the aqueous phase. For e.g.positively-charged alumina particles the adsorption may be carried outwith carboxylic acids in water at pH 4.75. By changing the anchoringpolar group of the amphiphile, the alumina surface can also be modifiedat alkaline pH conditions using for instance alkyl gallates as adsorbingmolecule. This amphiphile can also be used to lyophobize the surface ofa variety of metals and other amphoteric and basic oxides.Alternatively, the surface of acidic-like oxides such as silica, siliconcarbide and silicon nitride can be lyophobized employing amine-basedamphiphiles.

For the in-situ lyophobization of particles, the amphiphile is ingeneral applied in amounts of less than 1% by weight referred to theweight of the particles, preferably in amounts of <0.8% by weight. Theminimal amount of amphiphile that should be present, in general is about0.001%, preferably about 0.1%. Since the amphiphile—besides of otheringredients of the suspension—also influences the viscosity within theabove limits, the actual amount of modifier used is chosen dependent onthe desired final viscosity.

For some aspects of the present invention it is also possible to use perse hydrophobic particles, e.g. Teflon (polytetrafluoroethylene) or PVDF(polyvinyldifluoride) particles. Such particles need not be surfacemodified, but can be directly used in combination with solvents suitableto tune the particle lyophobicity such as e.g. water supplemented with ahydrophilic solvent, such as water/alcohol mixtures or water/glycolmixtures, in particular water/ethanol mixtures.

Further partially lyophobic/lyophobized particles can be obtained byincorporating hydrophilic groups or groups suitable to be coupled tohydrophilic groups into a polymer or polymer mixture that without saidgroups would result in the formation of hydrophobic particles.

Such particles can e.g. be made of polymeric core beads, e.g. beads ofpolystyrene, polymethyl methacrylate PMMA, with surface attachedcarboxylate or sulfonate groups.

It has been found that partially lyophobic/partially lyophobizedparticles with much different shapes can be used as foam stabilizers,i.e. particles that are spherical, polygonal plates, needles, fibres,rods, single crystals etc., provided that their particle size is withinsuitable dimensions. In addition, the particles themselves can be denseor porous or mixtures of dense and porous particles.

The mean particle size (measured for the largest dimension) can go up to100 μm for fibres and needles. In general, however, the mean particlesizes for all shapes are from 20 μm to 1 nm, preferably from 10 μm to 5nm, more preferably from 2 μm to 10 nm.

It has also been found that the particle size distribution is of lessimportance. Good foams can be obtained with narrow as well as with broadparticle size distributions.

The partially lyophobic or partially lyophobized particles are presentin amounts of at least about 1% by volume referred to the volume of thesuspension, preferably at least about 2% v/v, more preferred at leastabout 3% v/v, and still more preferred at least about 5% v/v. The upperlimit is provided by the viscosity that must not be too high. In generalsaid viscosity should not exceed 10 Pa·s at a shear rate of 100 s⁻¹.Slightly higher viscosities might be acceptable if very strong foamingapparatuses are available. The minimal amount needed to foam the wholesuspension depends on the particle size and can easily be determined bythe skilled person. In general the rule applies that the smaller theparticles are, the lower the minimally needed amount is.

The nature of the particles present will depend on the intended end useof the foam to be formed. It may be one or more of the followingexemplary materials: alumina, mullite, silicon carbide, silicon nitride,boron nitride, boron nitride, boron carbide, cordierite, silicas,zirconia, spinels, hydroxyapatite, calcium phosphates, in particulartri-calciumphosphate, cerium-gadoliniumoxide, magnesia and other metaloxides, e.g. tin oxide, titanium oxide and cerium oxide, or metal salts,e.g. nickel nitrate, nickel carbonate and the like, metals and alloys,such as ferrochrome, ferrosilicon, polymers, such as polyethylene,polytetrafluorethylene, polyvinylidenedifluoride. Multi-componentcompositions comprising mixtures of two or more of the above mentionedcompounds of the same or different kind may also be used.

As continuous phase, said liquid medium comprises a solvent selectedfrom the group comprising water, a hydrophilic solvent such as alcohols,glycols, etc. and mixtures thereof. Further additives can be added e.g.to adjust the pH, the ionic strength etc.

As already mentioned above, the preferred pH or pH range is dependent onthe headgroup used for in-situ lyophobization and the particle surfaceto be modified. It should be such that at least about 1.5%, preferablyat least about 10%, much preferred about 50% of the headgroups arenegatively charged (deprotonated) or positively charged (protonated).

The ionic strength can be adjusted to favour the close-packing of theattached particles at the interface and the attraction of particleswithin the foam lamella. However, the ionic strength should be kept lowenough to ensure sufficiently low viscosity of the suspension to allowsufficient introduction of air or good foaming, respectively, with theavailable apparatus.

According to the present invention, a suspension consisting of

(a) water and/or one or more hydrophilic solvents,

(b) (i) initially hydrophilic particles together with a hydrophobizingsurface modifier and/or in-situ partially hydrophobized particles,

-   -   -   (ii) initially hydrophobic particles together with a            hydrophilizing surface modifier and/or partially lyophilized            particles,        -   (iii) initially hydrophobic particles in a medium where the            surface tension is adjusted to adjust the particle            hydrophobicity (e.g. water/ethanol mixtures),        -   (iv) initially inherent partially hydrophobic particles, or        -   (v) combinations of two or more of (i) to (iv),

(c) optionally an acid or base to adjust the pH

(d) optionally an ionic strength influencing agent

wherein the particles are present in amounts of at least about 1% v/vreferred to the volume of the suspension, preferably about 2% v/v, morepreferred about 3% v/v, most preferred about 5% v/v,

is foamable and stable.

It is, however, also within the scope of the present invention, tosupplement such foamable and stable suspension with further additivessuch as polymers, surfactants, monomers (optionally together with apolymerization initiator or an active or latent curing agent), cement,chemicals suitable to release a gas under specific conditions such asH₂O₂ or N₂-releasing additives, or other ingredients known for foams.

The viscosity of the suspension preferably is such that the viscosity isless than the level at which the introduction of gas cannot take placeand above the level at which entrapped gas bubbles will tend to escape.

The critical viscosity of the suspension, in general, will be in therange of from about 5 mPa·s. to about 1000 mPa·s at a shear rate of 100s⁻¹, preferably 25 mPa·s to about 1000 mPa·s. The preferred range isdependent on the method of gas entrapment. For entrapment by mechanicalmeans e.g. stirring, the preferred range is 25 mPa·s to about 200 mPa·s.

The dispersed phase in general is a gas, in particular air. The samekind of stabilization, however is also applicable to stabilize dispersedhydrophobic liquid phases. Such hydrophobic liquid phases are e.g. fatsor oils or fats and/or oils comprising phases having e.g. essentialsubstances incorporated therein.

The foam can be prepared using different methods, for example byincorporating bubbles into the suspension. The incorporated bubbles maybe small bubbles, or they may be big bubbles that upon shearing of thesuspension are divided into the desired amount of small bubbles.

The bubbles of gas may be introduced in any convenient way. Forconvenience and economy the gas is air. Preferred methods ofintroduction include:

1. Subjecting the suspension to a high intensity and/or high speedagitation while exposed to the atmosphere. The agitation is preferablycarried out using a mixer, e.g. a mechanical mixer rotated at highspeed. The agitation is carried out for a sufficient period to introducebubbles of air into the dispersion until expansion has been achievedaccording to the desired physical and other properties of the endproduct. The expansion ratio, i.e. the volume of foam formed compared tothe volume of the starting suspension, can be between about 1.5 andabout 11, preferably between about 2 and about 7. The foaming of thedispersion may also be judged visually, i.e. because the foamedcomposition takes on the appearance of a meringue when sufficient airhas been introduced. Other gases which can be introduced includenitrogen, oxygen, argon and carbon dioxide;

2. The gas may be introduced by bubbling the gas through a filter of adefined pore size into the suspension while being stirred. In this casethe final pore size of the foam may be dependant on the pore size of thefilter;

3. In a variation, high pressure gas is forced through a fine filter,then intimately mixed with the suspension in a suitable chamber and theaerated mixture is then ejected from a nozzle;

4. The aerosol method may also be used, in this case the suspension ishoused in a pressurized vessel and gas is injected under pressure intothe suspension to produce a foam when ejected via a nozzle;

5. In another technique, a reactive gas generating substance may beadded to the suspension, the substance being selected to react with acidor alkali present with the suspension to produce the required gasin-situ, either when included or when subjected to agitation or heating.

The method for preparing foams of the invention is further characterizedby the following steps:

a) forming a suspension comprising as continuous phase water and/or oneor more hydrophilic solvents, and as dispersed phase

-   -   -   (i) initially hydrophilic particles together with a            hydrophobizing surface modifier and/or in-situ partially            hydrophobized particles,        -   (ii) initially hydrophobic particles together with a            hydrophilizing surface modifier and/or partially            hydrophilized particles,        -   (iii) initially hydrophobic particles in a medium where the            surface tension is adjusted to adjust the particle            hydrophobicity (e.g. water/ethanol mixtures,        -   (iv) initially inherent partially hydrophobic particles, in            particular a suspension as defined above, or        -   (v) combinations of two or more of (i) to (iv),

b) introducing gas into the suspension until the whole suspension ishomogeneously foamed; and, if a solid article shall be made,

c) removing the liquid carrier to provide a solid article having poresderived from the bubbles, and, optionally,

d) strengthening the structure e.g. by heat treatment up to the meltingpoint or the glass transition temperature or by sintering,

wherein the suspension has a critical viscosity selected to be below thelevel at which the introduction of gas cannot take place and above thelevel at which entrapped gas bubbles will tend to escape.

Using partially lyophobized or inherent partially lyophobic particles asfoam stabilizers allows the production of foams with an air content ofup to 95%, in general up to 90%, and preferred about 90%. The balance to100% is provided by the suspension comprising partially lyophobic orlyophobized particles and liquid medium/phase (continuous medium/phase).

The bubble size of the wet foam is dependent on all the aboveparameters, in particular the viscosity, the amount of additives, theamount of particles and—provided that no gas releasing chemical alone isused—the apparatus or the apparatus dependent method parameters used toget air into the suspension. The bubble size usually ranges from 1 μm to1 mm, preferably from 1 μm to 500 μm, more preferably from 1 μm to 50μm.

The foamed composition may be allowed or caused to acquire sufficientwet green strength to allow it to be moved from the container or mould.If not a wet foam but a solid article is desired, the composition may besubjected to drying. This step serves the removal of the solvent.

In the case of water the drying can be carried out at below about 100°C., e.g. in an oven or using high frequency drying equipment. The dryingstep may be varied. For example, the drying may be done under reducedpressure to cause the foam to expand before the green strength isdeveloped. The degree of expansion and hence the pore size of the foamwill depend on the pressure selected. Drying at elevated temperaturetends to cause a slight expansion of the foam. It is preferred tocontrol the humidity during the drying step, to prevent uneven shrinkageand drying cracks, whereas, if an optional polymerisable material ispresent in the dispersion, this step might not need to be taken.Temperature-assisted or vacuum-assisted unidirectional drying leads toan even shrinkage of the sample without inducing stresses which wouldresult in cracks.

As already mentioned above, the suspension may include otheringredients, e.g. ingredients which play a role at the drying stage.Examples of such ingredients include binders such as resins, e.g.polyvinylchloride, gums, cellulose, oligo and polysaccharides andpolymerisable materials to increase green strength. A specific class ofsuch additives is organic monomers such as soluble acrylates andacrylamides. The additives are preferably dissolved in deionized wateror other carrier liquid or a mixture to produce a premix solution, acatalyst is added to the dispersion before foaming and an initiatorafter foaming. Elevated temperature can be a suitable substitute for thecatalyst or both may be used together. Although the addition of bindersetc. in general is not needed for the inventive foams, such additivesmay have advantages if high green strength after drying is desired. Thebody formed in the presence of binders or polymerizable materials afterdrying is relatively robust, and the addition of binders orpolymerizable materials can be preferred when the article to be formedis of a complex shape.

In a similar way also stable emulsions can be made, namely by a methodcomprising the following steps:

a) forming a suspension comprising as continuous medium water and/or oneor more hydrophilic solvents, and as dispersed phase

-   -   -   (i) initially hydrophilic particles together with a            hydrophobizing surface modifier and/or in-situ partially            hydrophobized particles,        -   (ii) initially hydrophobic particles together with a            hydrophilizing surface modifier and/or partially            hydrophilized particles,        -   (iii) initially hydrophobic particles in a medium where the            surface tension is adjusted to adjust the particle            hydrophobicity (e.g. water/ethanol mixtures),        -   (iv) initially inherent partially hydrophobic particles, or        -   (v) combinations of two or more of (i) to (iv),

b) introducing at least one non-polar liquid or mixtures of non-polarliquid and gas into the suspension until the whole suspension is formedinto a homogeneous emulsion;

wherein the suspension has a critical viscosity selected to be below thelevel at which the introduction of non-polar liquid or mixtures ofnon-polar liquid and gas cannot take place and above the level at whichoptionally present entrapped gas bubbles will tend to escape and oildroplets will tend to phase separate.

Subsequent processing will depend on the nature of the intended articleand the materials used; examples of suitable steps include shaping, e.g.machining, sintering at usual sintering temperatures, e.g. at 1400° C.to 1600° C. for Al₂O₃, impregnation of the pores with, e.g. catalystsand/or other agents. Porous articles made according to the invention caninclude: catalyst supports, flame supports and arresters; gas filters;airfresheners, ceramic armour; diesel particulate traps; insulationmaterials; artificial parts for the body; metal filters, reusablefilters; liquid filters; Storage and transportation for flammable and/ortoxic materials, humidity sensors, chromatography, filter candles forfiltration of hot combustion gases, diaphragms, membranes, refractoryseparators, phase dividers and electrolytes for high temperature fuelcells.

The solidifying steps described above can also be applied to emulsions.

It is also possible to dilute wet-foams and emulsions prior to dryingsuch that separate shells or capsules are formed, the structure of whichcan be conserved by appropriate drying and optional sintering.

In the case of shell or capsule production, additional additives arepreferably added such as binders, surfactants, alcohols etc. to avoidcollapsing of the shells or capsules.

Such shells or capsules out of particles that contain air or non-polarliquids or mixtures thereof can be made by a method comprising thefollowing steps:

a) forming a suspension comprising as continuous medium water and/or oneor more hydrophilic solvents, and as dispersed phase

-   -   -   (i) initially hydrophilic particles together with a            hydrophobizing surface modifier and/or in-situ partially            hydrophobized particles        -   (ii) initially hydrophobic particles together with a            hydrophilizing surface modifier and/or partially            hydrophilized particles        -   (iii) initially hydrophobic particles in a medium where the            surface tension is adjusted to adjust the particle            hydrophobicity (e.g. water/ethanol mixtures) or        -   (iv) initially inherent partially hydrophobic particles        -   (v) combinations of two or more of (i) to (iv),

b) introducing gas or at least one non-polar liquid or mixtures ofnon-polar liquid and gas into the suspension until the whole suspensionis formed into a homogeneous foam or emulsion,

c) diluting the foam or emulsion by means of adding additional liquid,in particular water and/or one or more hydrophilic solvents

d) drying the shells or capsules in air or in particular by means offreeze drying, super critical drying or similar methods.

The characteristics of the end product may be varied according to theconditions under which the method is performed. Where the contents ofthe solids in the dispersion are low, the viscosity will be reduced butthe foam stability may be affected; lower viscosity dispersions tend toyield articles of lower density, i.e. higher pore content for a givensolids content. By increasing the speed of stirring when introducing thegas bubbles the article formed will have a high pore content and a fineraverage pore size.

The pore size of the porous article is dependent on all the aboveparameters, in particular the viscosity of the suspension, the amount ofadditives, the amount of particles and—provided that no gas releasingchemical alone is used—the apparatus or the apparatus dependent methodparameters used to get air into the suspension. It usually ranges from 1μm to 1 mm, preferably from 1 μm to 500 μm, more preferably from 1 μm to50 μm.

Using partially lyophobized particles or inherently partially lyophobicparticles as foam stabilizers allows the production of porous articleswith porosities of up to 95%, in general up to 90%, and preferred about90%.

It is a feature of the invention that in case of solid articleproduction, in particular in the absence of optional additives, thefinal articles formed consist essentially of the starting refractorymaterials only. By using the inventive wet foams in the production ofsolid articles, not only the need for the presence of residual secondarymaterials, e.g. inorganic binders, can be avoided but also a weakeningof the porous solid structure due to the decomposition of an organicadditive during the sintering/firing process. Additionally, the poresize remains small due to the long-term stability in the wet state.

For many applications, the use of initially lyophilic, in particularhydrophilic instead of lyophobic, in particular hydrophobic particles ispreferred since they can be easily dispersed in the liquid phase beforethe foaming process thereby avoiding the formation of agglomerates. Thepresent invention made it for the first time possible to stabilize foamsby using a large variety of particles thereby enabling not only foamswith desired stability but simultaneously the adaptation of the chemicalcomposition to specific desires. For example in the case of foodindustry, essential elements in nanoparticulate form can in desiredcomposition be used to produce a foam. Or in the cosmetics industry forexample physical UV filters can be incorporated into foams ordispersions with the benefit that they simultaneously stabilize saidformulations. Alternatively, such foams can also be used as dye vehiclein textile industry, to suppress explosions or as fire extinguisher.

The high stability achieved with the inventive method enables thedevelopment of numerous new products in food and cosmetics industries,as well as in engineering, chemical and medical fields. The fabricationprocess of such porous materials including shells and capsules isextremely simple and inexpensive, and allows to produce porous ceramicstructures with a wide range of compositions, porosity and pore sizedistributions that can be used as thermal and electric insulatingceramic parts, scaffolds and biomedical implants, porous electrodes forsolid oxide fuel cells, as well as low dielectric constant materials forelectronic applications; filters for molten metals and hot or corrosivegases; catalyst carriers; fire extinguisher.

Shells and capsules obtainable by the inventive methods can ab-initio orsubsequently be loaded with a broad variety of gaseous, liquid or solidcompounds, also very sensitive compounds, such as drugs, biomolecules,cells, in particular living cells, fragrances, flavors, dyes etc.

Due to the long time stability of the foams and the high air content,the method of the present invention is especially suitable to producelight weight ceramics, wherein also mixed ceramics are easilyobtainable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those setforth above will become apparent when consideration is given to thefollowing detailed description thereof. Such description makes referenceto the annexed drawings, wherein:

FIG. 1 is a schematic drawing of different approaches for foamformation, wherein 1-3 concern particles that are partially lyophobizedeither through surface modification starting from hydrophilic particlesthat are partially hydrophobized (1) or from hydrophobic particles thehydrophobicity of which has been reduced (3) or through the modificationof the surface tension of the liquid media (2), and (4) concernsparticles that are per se partially lyophobic and need no modification.

FIG. 2 shows possible approaches to tune the wetting properties and thelyophobic nature of colloidal particles, wherein

(a) shows the adsorption of partially lyophobic particles at thegas-liquid interface, illustrating the tension (γ) balance responsiblefor particle attachment,

(b) shows the approaches used to tune the wetting properties oforiginally hydrophobic particles and hydrophilic particles to illustratethe universality of the foaming method developed.

and (c) shows the approaches used to tune the wetting properties ofpartially hydrophobized, initially hydrophilic particles. FIG. 1illustrates the universality of the foaming method developed. The sameprinciples can be easily extended to other types of particles, usingdifferent surface modifiers, liquid and gaseous phases.

FIG. 3 shows an example of surface lyophobization using differentsurface modifiers, liquid and gaseous phases, and foaming behavior usingalumina particles and short fatty acids as amphiphiles, wherein all datawere obtained from 35 vol % alumina suspensions at pH 4.75, and wherein

(a) shows the surface lyophobization of colloidal particles accomplishedthrough the electrostatic-driven adsorption of negatively-chargedcarboxylic acids on positively-charged alumina particles,

(b) shows the ability of lyophobized particles to attach at air-waterinterfaces which results in a significant decrease in the surfacetension of colloidal suspensions, and

(c) shows that such decrease in surface tension resulted in remarkablyhigh foamability upon high mechanical shearing.

FIG. 4 shows the hierarchical features of the particle-stabilized foamscontaining short amphiphilic molecules. High-volume macroscopic foams(a) with bubble size within the range 10-50 μm (b) are formed throughthe adsorption of submicron-sized colloidal particles at the air-liquidinterface (c). A possible arrange of the colloidal particles andamphiphilic molecules at the air-water interface is schematicallyillustrated in (d) for particles partially covered with the amphiphiles.The confocal images shown in (b) and (c) were obtained after dilution ofconcentrated foams (inset in b) containing fluorescently-labelled silicaparticles and hexyl amine as amphiphile.

FIG. 5 shows the outstanding stability of particle-stabilized foamsprepared with alumina and valeric acid at maximum foamability. Nodrainage and disproportionation was observed in all particle-stabilizedfoams prepared with partially lyophobic particles (exemplified in d), ascompared to the considerable destabilization that takes place inwell-established cosmetic foams (a, shaving foam) and food foams (b,whipped cream; c, egg white foam). Images shown on the left-hand sidewere taken 5 minutes after foaming, whereas those on the right-hand sidewere taken after 4 hours for the shaving (a), 69 hours for the ),whipped cream (b), 67 hours for the egg white (c) and 100 hours for theparticle-stabilized foams (d). Experiments for comparison with shortalkylamines adsorbed at the air-water interface and a high concentration(35 vol %) of fully hydrophilic alumina particles in the bulk phaserevealed that the foam stability is not caused by the well-knownapproach of increasing the viscosity of the foam lamella. The same wasshown in experiments with short carboxylic acids adsorbed at theair-water interface and a high concentration (35 vol %) of coagulatedfully hydrophilic silica particles in the bulk phase.

MODES FOR CARRYING OUT THE INVENTION

In FIG. 1, the different methods to arrive at partiallylyophobic/partially lyophobized or partially hydrophobic/partiallyhydrophobized particles is illustrated. The numeral 1 designates themodification starting from initially lyophilic/hydrophilic particlesthat—due to the reaction with a surface modifier, e.g. an amphiphile,are rendered less hydrophilic or that are partiallylyophobized/hydrophobized. Numeral 3 designates the modificationstarting from the opposite direction, i.e. the modification startingfrom initially lyophobic/hydrophobic particles that—due to the reactionwith a surface modifier are rendered less hydrophobic or that arepartially lyophilized/hydrophilized. Numeral 2 refers to themodification starting from initially lyophobic/hydrophobic particles ina medium where the surface tension is adjusted to adjust the particleslyophobicity, and numeral 4 refers to particles that are initiallyinherent partially lyophobic/hydrophobic particles and need nomodification.

In the following description, the invention is further described basedon one kind of hydrophilic particles, namely alumina particles. However,ultrastable high-volume foams containing a variety of metal oxideparticles and different short amphiphilic molecules have been preparedusing the below further described novel approach. This approach thus isby no means limited to the examples herein described. New foamformulations using other particles and other surface modifiers, inparticular amphiphiles, can be prepared applying the concepts outlinedabove and here.

Aqueous foams containing alumina submicron-sized particles and a seriesof short-chain carboxylic acids were chosen as a modelparticle-amphiphile system to illustrate the new method.

The primary requirement for foam formation is the attachment ofparticles to the gas-liquid interface. It has been found that this canbe achieved by deliberately tuning the wetting properties and thelyophobicity of the particle surface. A contact angle of 20° to 90° hasproved to be in general suitable. The examples herein describedillustrate the universal nature of the method, which in principle can beextended to any type of particles regardless their initial wettingbehavior.

In general, the surface of synthetic and natural colloidal particles isoften either predominantly hydrophilic or hydrophobic in nature.Hydrophilic particles (e.g. oxides) are completely wettable in water, asopposed to the non-wetting features of typically hydrophobic particles(e.g. polymers and fats). In order to tune the particle hydrophobicityand induce their adsorption at air-water interfaces, wetting onhydrophilic particles has to be diminished, whereas wetting onhydrophobic particles ought to be enhanced. FIG. 2 shows some of theapproaches used as examples of how the wetting properties of typicallyhydrophobic and hydrophilic particles can be tuned in order to favortheir adsorption at the gas-liquid interface.

Partial wetting of originally hydrophobic particles can be achieved byadjusting, for instance, the composition of the liquid aqueous phaseusing water/alcohol mixtures (FIG. 2( b)). Alternatively, polar orionizable groups can be physically or chemically grafted on the particlesurface to partially enhance its wettability in water.

For hydrophilic particles, on the other hand, partial surfacehydrophobization can be accomplished is through the adsorption ofshort-chain amphiphilic molecules on the solid-liquid interface, asillustrated in the examples shown in FIG. 2( c). By choosing appropriateanchoring groups and pH conditions, a wide variety of particles can besurface hydrophobized through the adsorption of short amphiphiles viaelectrostatic interactions and ligand exchange reactions.Hydrophobization occurs due to the relatively strong interaction betweenthe anchoring group and the particle surface, leaving the amphiphilehydrophobic tail in contact with the aqueous solution. This approachresembles that applied for the separation of micron-sized tomillimeter-sized ore particles in flotation processes, using amphiphilescontaining typically more than 10 carbons in the hydrophobic tail [15].It is important to note that in the case of submicron sized colloidalparticles in concentrated suspensions, the high total surface area ofsolids requires the use of amphiphilic molecules that exhibit highsolubility and critical micelle concentrations in the aqueous phase inorder to impart a substantial coverage of the particle surface.Therefore, molecules with typically less than 8 carbons on thehydrocarbon tail were employed for surface hydrophobization.Alternatively, partial hydrophobicity can also be imparted by chemicallyattaching alkyl silanes on the particle surface [8, 9, 14].

FIG. 3( a) shows an example of the electrostatic-driven adsorption ofanionic carboxylate amphiphiles onto positively-charged aluminaparticles in water at acidic pH conditions (FIG. 2( c)). Thehydrophobization achieved via amphiphile adsorption was confirmed bycontact angle measurements of valeric acid aqueous solutions (0.05mol/L; pH=4.75) deposited on polycrystalline alumina substrates, whichrendered angles of approximately 60° measured through the aqueous phase.The attachment of the resulting partially hydrophobic particles at anair-water interface was indirectly evidenced by surface tensionmeasurements of a suspension droplet at various concentrations of addedamphiphilic molecules (FIG. 3( b)). The gradual decrease in surfacetension observed at lower amphiphile concentrations is caused by theadsorption of free bulk amphiphilic molecules at the gas-liquidinterface. Higher amphiphile concentrations, on the other hand, led to asteep reduction of surface tension as a result of the adsorption of thecoated particles at the interface. The adsorption of hydrophobizedparticles into the air-water interface was also evidenced by theformation of a thin stiff skin on the surface of these suspensions.

Extensive surface lyophobization may however lead to strong coagulationbetween particles within the liquid media. Coagulation results from theaction of van der Waals and hydrophobic attractive forces amongpartially lyophobized particles. Although the exact origin of surfacehydrophobic forces is still a matter of controversy in the literature[16, 17], the hydrophobic attractive effect has been clearlydemonstrated in both flat and curved surfaces [16, 18, 19]. In the caseof charged particles, repulsive Derjaguin, Landau, Vervey and Overbeek(DLVO) forces can to some extent prevent strong coagulation, aiding theincorporation and dispersion of particles into the colloidal suspensionprior to the foaming process.

The low surface tension achieved via the adsorption of hydrophobizedparticles at the air-water interface (FIG. 3( b)) enables thepreparation of foams by simply incorporating air bubbles throughmechanical shearing, internal gas expansion or gas-releasing chemicalreactions within the colloidal suspension. Foams prepared by vigorousmechanical shearing of concentrated alumina suspensions (35 vol %solids), for instance, showed a fivefold to sixfold increase in volumeat optimum concentrations of carboxylic acid, as illustrated in FIG. 3(c). This volume increase corresponds to an amount of incorporated air ofapproximately 85% with respect to the total foam volume. A bubble sizedistribution ranging typically from 10 to 100 μm is formed via thisfoaming process at maximum foamability conditions. Narrower bubble sizedistributions are achieved by increasing the particle hydrophobicity.Further increase of the surface hydrophobicity leads, however, toextensive particle coagulation in the suspension, hindering the foamingprocess. This general foaming behavior was observed for all the examplesoutlined in FIG. 2. The surprisingly high foamability achieved with thisnew approach in comparison to previous investigations onparticle-stabilized bubbles [8-10, 14, 20] is related to the propertuning of some key aspects involved during the process of foam formationas outlined herein.

Foam formation is a dynamic non-equilibrium process which dependsstrongly on the kinetics of the diffusion-limited adsorption of surfaceactive species on the air-water interface. High-volume foams aretypically obtained when the time needed for the diffusion of surfaceactive species to the interface is shorter than the lifetime of freshlycreated bubbles. The time required for particles to adsorb on the bubblesurface can be reduced by increasing the particle diffusion coefficientor by decreasing the distance between the particle and the air-waterinterface. According to the Stokes-Einstein relation, the diffusioncoefficient is inversely proportional to the particle size. Theparticle-interface distance, on the other hand, is inverselyproportional to the number concentration of particles in the aqueousmedia. Based on these considerations, foam formation is favored byincreasing the concentration of particles or decreasing the size ofadsorbing particles. During foaming, the coagulation of single particlesinto clusters can markedly increase the diffusion time to the bubblesurface, due to an increase of the size and a reduction of the numberconcentration of surface active species. Since particle coagulation isfavored with the increase of surface hydrophobicity, extensivehydrophobization hinders foam formation due to the build-up of massiveparticle clusters in the aqueous phase, as recently outlined byDickinson et al. [9]. High surface hydrophobicity and low concentrationof particles (<2 vol %) were probably the reasons for the limitedfoamability achieved in previous investigations on particle-stabilizedair bubbles [8-10, 20]. Even though particles exhibiting contact angleclose to 90° lead to a favorable irreversible adsorption at air-liquidand liquid-liquid interfaces [14], the results shown here indicate thatenhanced foamability is achieved using high concentrations of slightlyhydrophobized particles which do not extensively coagulate in theaqueous phase and are thus able to promptly adsorb on the air bubblesurface. For the particle size used in the example reported in FIG. 3(diameter˜200 nm), a minimum colloid concentration of 15 vol % wasnecessary to obtain relatively stable high-volume foams. However, thislower concentration limit could be reduced to approximately 5 vol % byusing highly mobile partially-hydrophobized nanoparticles (diameter˜30nm) as foam stabilizers.

The adsorption of hydrophobic particles at the air-water interface ofthe inventive foams was confirmed by confocal microscopy images of airbubbles obtained from the dilution of concentrated fluorescent silicafoams. An enormous number of extremely stable air bubbles or hollowcolloidosomes [21] were produced upon foam dilution, as shown in FIG. 4.Small clusters of particles were adsorbed at the air-water interface,suggesting the existence of an attractive colloidal network around theair bubbles. In this particular example, particles are positioned veryclosely to the bubble surface but apparently not pushed into theair-water interface (FIGS. 3( c) and (d)). It is quite probable thatparticles hydrophobized via the surface adsorption of amphiphiles,behave slightly different at the air-water interface in comparison tothe idealized situation depicted in FIG. 2( a). Due to their highmobility at the particle surface, the amphiphiles may in this caseaccumulate on the surface area close to the neighboring air bubble,using the particles as a template for their adsorption at the air-waterinterface (FIG. 4( d)). This leads to a substantial decrease in surfacetension without necessarily pushing the particles into the interface, aswould be expected for homogeneously hydrophobized particles (FIG. 2(a)). It shall be distinctly understood that this is only a hypothesisthat by no means is intended to limit the scope of the presentinvention.

The stability of the high-volume particle-stabilized foams was comparedto that of foams known to be very stable in cosmetic and foodapplications. No liquid drainage and bubble disproportionation wasobserved in particle-stabilized foams within more than 4 days after foampreparation, as shown in FIG. 5. This outstanding stability contrasts tothe markedly higher drainage and disproportionation rates of theevaluated food and cosmetic foams. Liquid foams containing conventionallong-chain surfactants adsorbed at the air-water interface collapse muchfaster than the foams investigated here, typically within a couple ofminutes.

Among the several mechanisms leading to foam destabilization [22],bubble disproportionation had so far been particularly difficult toavoid in liquid foams due to the ever-present difference in Laplacepressure between bubbles of distinct sizes, which ultimately results ina steady diffusion of gas molecules from smaller to larger bubbles overtime [22]. The remarkable resistance of particle-stabilized foamsagainst coalescence and disproportionation is most likely imparted bythe strong attachment of particles at the air-water interface (FIG. 4)and by the formation of an attractive particle network at the interfaceand throughout the foam lamella.

Particles attached to the air-water interface can reduce the overallfoam free energy by thousands of kTs, if a considerable amount ofinterfacial area is replaced upon adsorption [14, 23]. Such a reductionin free energy makes the interfacial adsorption of partially-hydrophobicparticles an irreversible process, as opposed to the continuousadsorption and desorption of conventional surfactant molecules at theair-water interface (Gibbs-Marangoni effect). Particles stronglyadsorbed at the interface may resist the shrinkage of small bubblesduring disproportionation by forming a percolating interfacial armorthat mechanically withstands the low pressures resulting from gasdiffusion outwards the bubble [11]. The fact that the air bubbles arehighly confined throughout the foam volume may also contribute to theenhanced stability, by restricting the movement of particles attached tothe interface. In this case, the immobile attached particles wouldsignificantly hinder the mobility of the air-water interface, resemblingthe well-known pinning effect of particles in grain boundaries ofpolycrystalline materials [24].

Wet foams with remarkable long-term stability and bubble size as smallas 10-100 μm can be prepared for cosmetic and food applications usingthe described method. The strong attachment of particles at theair-water interface also enables the fabrication of an enormous numberof hollow colloidosomes for a variety of emerging applications [21].Additionally, the outstanding foam stability allows to fabricatemacroporous structures with a variety of different ceramic, polymericand metallic materials by drying and heat treating the wet foams.Macroporous materials prepared by this simple and straightforward methodcan be used as low-weight structural components, porous media forchemical and biological separation, thermal and electrical insulatingmaterials, catalyst supports, refractory filters for molten metals, andscaffolds for tissue engineering and medical implants [25-27].Therefore, this novel technique aids the development of new products ina wide number of areas, including food, cosmetics, engineering, biologyand medicine.

In the following text some Examples of wet particle-stabilized foams andemulsions are given, as well as Examples for the production of dried andsintered porous articles:

EXAMPLE I Al₂O₃ Foam at Acidic pH

A slurry comprising 50 vol % alumina powder (Ceralox) with a meanparticle size of 200 nm was prepared by adding the powder stepwise to asolution containing water and 2.8 wt % 2N HCl (to alumina).Homogenization took place on a ballmill during 20 hours.

After ballmilling, short-chain carboxylic acids (tail length between 2and 6 carbons) were added to the suspension and the pH was set to 4.75with either 2N HCl or 1N NaOH. The desired solids loading (typically 35vol %) was achieved by diluting the suspension with additional water.

This suspension was then mixed with a Kenwood kitchen mixer for 3 min (1min at speed 4 and 2 min at maximum speed) to obtain the foam.

Investigation on the appearance and the behaviour of these foams areshown in FIG. 5, wherein FIG. 5 shows a typical light microscope imageof a wet alumina foam.

EXAMPLE II Al₂O₃ Foam at Basic pH

A slurry comprising 50 vol % alumina powder (Ceralox) with a meanparticle size of 200 nm was prepared by adding the powder stepwise to asolution containing water and 29 mmol/l propyl gallate. Homogenizationtook place on a ballmill during 20 hours. After ballmilling, the propylgallate concentration was adjusted to 100 mmol/l and the pH was set to9.8 with either 2 N HCl or 1 N NaOH. A solids loading of 35 vol %alumina was achieved by diluting the suspension with additional water.

This suspension was then mixed with a Kenwood kitchen mixer for 5 min (1min at speed 4 and 4 min at maximum speed) to obtain the foam.

EXAMPLE III SiO₂ Foam

A slurry comprising 50 vol % silica powder (Nissan) with a mean particlesize of 70 nm was prepared by stepwise adding the powder to the water.Homogenization took place on a ballmill during 40 hours. Afterballmilling, 66 mmol/l hexylamine was added and the pH was set to 9.8with either 2N HCl or 1N NaOH. A solids loading of 35 vol % was achievedby adding water. The suspension was then mixed with a Kenwood KitchenMixer during 1 min at speed 4 and 2 min at the maximum speed to obtainthe foam.

EXAMPLE IV ZrO₂ Foam

A slurry comprising 50 vol % zirconia powder (Tosho) with a surface areaof 15.2 m²/g and 100 mmol/l propyl gallate was prepared by stepwiseadding the powder to a solution of water and propyl gallate.Homogenization took place on a ballmill during 20 hours. After ballmilling, the pH was set to 9.8 with 2N HCl. A solids loading of 20 vol %was achieved by adding water. The suspension was then mixed with aKenwood Kitchen Mixer during 1 min at speed 4 and 6 min at the maximumspeed to obtain the foam.

EXAMPLE V Ca₃PO₄ Foam

A slurry comprising 10 vol % tri-calcium phosphate (TCP) with a surfacearea of 33 m²/g and 150 mmol/l propyl gallate was prepared. The pH wasset to 9.5 with 4 N NaOH. The suspension was then mixed with a KenwoodKitchen Mixer during 1 min at speed 4 and 6 min at maximum speed toobtain the foam.

EXAMPLE VI Ce_(0.8)Gd_(0.2)O₂ Foam

A slurry comprising 10 vol % Ce_(0.8)Gd_(0.2)O₂ (CGO20) with a meanparticle size of 500 nm and 15 mmol/l valeric acid was prepared. The pHwas set to 4.75 with either 2 N HCl or 1 N NaOH. The suspension was thenmixed with a Kenwood Kitchen Mixer during 1 min at speed 4 and 2 min atmaximum speed to obtain the foam.

EXAMPLE VII Al₂O₃/Octane Emulsion

A slurry comprising 50 vol % alumina powder (Ceralox) with a meanparticle size of 200 nm was prepared by adding the powder stepwise to asolution containing water and 2.8 wt % 2 N HCl (to alumina).Homogenization took place on a ballmill during 20 hours. Afterballmilling, 131 mmol/l propionic acid was added to the suspension andthe pH was set to 4.75 with either 2 N HCl or 1 N NaOH. A solids loadingof 35 vol % was achieved by diluting the suspension with additionalwater. Octane was then added to the suspension to achieve aconcentration of 80 vol % of oil. This mixture was emulsified by mixingit with a Kenwood Kitchen Mixer during 1 min at speed 4 and 3 min atmaximum speed.

EXAMPLE VIII Polyvinylidene Fluoride Foam

Polymer particulate material (Polyvinyliden Fluoride) with MW 140 000was mixed with water/ethanol mixture (85/15). A solids loading of 15 wt% was achieved. The suspension was mixed with a Kenwood Kitchen Mixerduring 1 min at speed 4 and 6 min at the maximum speed to obtain thefoam.

EXAMPLE IX Foam from Partially Hydrophobized Silica Particles

Partially hydrophobized silica particles (SiO₂ HDK H30, 250 m²/g, 50%SiOH on the surface) were mixed with water containing 5 vol % ofethanol. A solids loading of 5 wt % was achieved. The suspension wasmixed with a Kenwood Kitchen Mixer during 1 min at speed 4 and 6 min atthe maximum speed to obtain the foam.

Example of Porous Ceramics from Wet Particle-Stabilized Foams

Foams were produced as described in example I.

Drying of the Wet Foams

The high stability of the wet foams allows for long drying times.Therefore, different methods can be used to dry the wet foams:

-   -   freeze drying    -   drying in air    -   unidirectional drying on a hot plate (80° C.) under controlled        environmental conditions (air temperature: 28° C., humidity:        80%)    -   unidirectional drying on a porous gypsum mold under vacuum    -   slip casting in a porous gypsum mold

Sintering of the Dried Foams

The dried foams are sintered at 1575° C. for 2 hours. The heating rateis 1° C./min, whereas the cooling rate is set to 3° C./min.

Properties of the Sintered Foams

FIG. 10 shows a picture of an alumina foam which was dried by slipcasting and sintered as mentioned above resulting in a porous ceramicpiece with 90% porosity.

Different properties such as the density, the pore size, the compressivestrength, the thermal conductivity as well as the dielectric constantwere measured and are summarized in Table 2.

TABLE 2 Properties of sintered alumina foam Average Unidirectional poreCompressive Thermal Dielectric drying Density Porosity size strengthconductivity constant method [g/cm³] [%] [μm] [MPa] [W/mK] [—]temperature 0.438 89.0 30 5.42 2 1.4 assisted vacuum assisted 0.505 87.320 16.3 — —

While there are shown and described presently preferred embodiments ofthe invention, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpracticed within the scope of the following claims.

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The invention claimed is:
 1. A method of preparing wet foams exhibitinglong-term stability comprising the steps of a) forming a suspensioncomprising as continuous medium water and/or one or more hydrophilicsolvents and colloidal particles as a dispersed phase and b) introducinggas into the suspension to produce a wet foam wherein said colloidalparticles are used to stabilize a gas-liquid interface and wherein theparticles are present in amounts of at least about 5% by volume referredto the volume of the suspension, wherein the particles are initiallyhydrophilic and are in-situ lyophobized through adsorption ofamphiphilic molecules on the particle surface, wherein amphiphilicmolecules consist of a tail coupled to a headgroup, wherein the tail isan aliphatic C₂-C₈ (linear or branched) or cyclic (alicyclic oraromatic) optionally substituted hydrocarbon and the headgroup is anionic group selected from phosphates, phosphonates, sulfates,sulfonates, amines, amides, pyrrolidines, gallate, carboxylic acids, andcorresponding salts, and wherein the whole of the suspension ishomogeneously foamed wherein the foam has a long term stability of atleast one week.
 2. The method of claim 1 wherein the mean particle sizes(measured for the largest dimension) are up to about 100 μm for fibersand needles.
 3. The method of claim 1 wherein the mean particle sizes(measured for the largest dimension) are from 20 μm to 1 nm for allparticle shapes except fibers and needles.
 4. The method of claim 1wherein the amphiphilic molecules suitable for in-situ surfacehydrophobization are able to reduce surface tension of an air-waterinterface to values less than or equal to 65 mN/m for concentrationsless than or equal to 0.5 mol/l and wherein the amphiphilic moleculeshave a solubility in the liquid phase of the suspension given by thefollowing equation:${{SOLUBILITY}\left\lbrack {{mol}\text{/}l} \right\rbrack} \geq {m \cdot \frac{\phi}{1 - \phi} \cdot \rho_{powder} \cdot S_{A}}$m = 4 ⋅ 10⁻⁸(T^(s)[mol/rn²]ϕ:  Solids   Loading  [−]ρ_(power):  Density  of   powder   [g/l]S_(A):  Surface  Area  of  Powder   [m₂/g].5. The method of claim 1 wherein the tail comprises an optionallysubstituted linear carbon chain of 2 to 8 carbon atoms, and theheadgroup is selected from the group consisting of:

and corresponding salts.
 6. The method of claim 1, wherein the pH duringin-situ lyophobization is such that at least about 1.5% of theheadgroups are negatively or positively charged.
 7. The method of claim1 wherein the suspension further comprises initially hydrophobicparticles.
 8. The method of claim 7 wherein the amphiphilic moleculessuitable for in-situ surface lyophilization have a critical micelleconcentration (CMC) higher than 10 μmol/l or have a solubility higherthan 1 μmol/l or where the particles lyophobicity is adjusted throughadjustments of the liquid medium or continuous phase.
 9. The method ofclaim 1 wherein for the in-situ lyophobization of particles, a modifieris applied in amounts of 0.001 to 1% by weight referred to the weight ofthe particles.
 10. The method of claim 1 wherein the particles areselected from the group consisting of oxides, carbides, nitrides,phosphates, carbonates, polysaccharides, salts, metals, polymers, fatsand mixtures thereof.
 11. The method of claim 1 wherein the suspensionfurther comprises pH and/or ionic strength adjusting agents.
 12. Themethod of claim 1 wherein the suspension further comprises additivesselected from the group consisting of polymers, surfactants, monomersoptionally together with a polymerization initiator or an active orlatent curing agent.
 13. The method of claim 1 comprising the followingsteps: a) forming a suspension comprising as continuous medium waterand/or one or more hydrophilic solvents, and as dispersed phase saidparticles and optionally one or more of (i) initially hydrophobicparticles together with a hydrophilizing surface modifier and/orpartially lyophilized particles, (ii) initially hydrophobic particles ina medium where the surface tension is adjusted to adjust the particlehydrophobicity, and (iii) initially inherent partially hydrophobicparticles b) introducing gas into the suspension until the wholesuspension is homogeneously foamed; wherein the suspension has acritical viscosity selected to be below the level at which theintroduction of gas cannot take place and above the level at whichentrapped gas bubbles will tend to escape.
 14. The method of claim 9,wherein the modifier is applied in amounts of 0.1 to 0.8% by weight. 15.The method of claim 1, wherein the suspension further comprisesadditives selected from the group consisting of reactive gas generatingsubstances, said substance being selected to react with acid or alkalipresent in the suspension to produce said gas in situ.
 16. The method ofclaim 12, wherein the suspension further comprises additives selectedfrom the group consisting of reactive gas generating substances, saidsubstance being selected to react with acid or alkali present in thesuspension to produce said gas in situ.